This paper explores a sketch-based interface for quickly yet accurately creating visually realistic hair for virtual characters. Recently, physically-based models have proved successful for generating a wide variety of hair types, but they do not provide a straightforward method for designing target hairstyles. The contribution of this paper is to propose a user-friendly method for controlling such a physically-based model, requiring no specific knowledge of mechanics or hair styling: the user sketches example hair strands over a side view of the character?s head, or alternatively annotates a picture of real hair viewed from the side serving as a reference. We show how the sketch can be used to infer the geometric and mechanical properties of the hair strands, to adjust the shape of the scalp, and to generate an adequate hair volume. Our method is validated on a wide variety of hair styles, from straight to curly and short to long hair.